Television camera tube in which the detrimental effect of the return beam is counteracted

ABSTRACT

A television camera tube (vidicon) comprising a cylindrical electrode with a diaphragm to restrict the cross-section of the electron beam. In order to prevent the detrimental effect of the return beam, the diaphragm has the shape of a truncated cone.

[451 Feb. 11,1975

United States Patent 11 1 Schut [54] TELEVISION CAMERA TUBE IN WHICH 2,839,601 6/1352 Fries 360/23 TH DETRIMENTAL EFFECT OF THE 3,183,400 5 1 6 Jensen et al. 313/387 RETURN BEAM 1S COUNTERACTED 3,307,061 2/1967 Schlesinger.. 313/387 [76] Inventor:

Theodore G. Schut, Emmasingel, Eindhov n, N the l d Primary ExaminerRobert Segal Mar. 4, 1974 Attorney, Agent, or FirmFrank R. Trifari; Carl P. Steinhauser [22] Filed:

[21] Appl. No; 447,563

Related US. Application Data ABSTRACT A television camera tube (vidicon) comprising a cylin- [63] Continuation of Ser. No. 265,259, June 22, 1972,

abandoned.

5 3 9 313/390 drical electrode with a diaphragm to restrict the cross- 51 lm. H0lj 31/38, HOlj 25/00 section of electron beam Order to Prev?nt the [58] Field of Search..................... 313/382, 389, 390 detrimental effect of the return beam; the diaphragm has the shape of a truncated cone.

1 Claim, 1 Drawing Figure [56] References Cited UNITED STATES PATENTS 2,747,133 5/1956 Weimer 315/11 TELEVISION CAMERA TUBE IN WHICH THE DETRIMENTAL EFFECT OF THE RETURN BEAM IS COUNTERACTED This is a continuation, of application Ser. No. 265,259, filed June 22, 1972 now abandoned.

The invention relates to a television camera tube comprising, centered along an axis, an electron gun and a photoconductive layer which is provided on a transparent, conductive-signal layer, on which photoconductive layer a potential distribution is formed on the surface which as not in contact with the signal layer by projecting an optical image on the photoconductive layer, which potential distribution is periodically reduced to the potential of the cathode by scanning the photoconductive layer with an electron beam produced by the electron gun, said tube furthermore comprising a hollow cylindrical electrode in which a diaphragm is provided having a final aperture, taken in the direction of the electron gun to the photoconductive layer, which restricts the cross-section of the electron beam.

Such a television camera tube is generally known and is termed vidicon. The operation of a vidicon is as follows: Under the influence of the deflection fields an electron beam ofa sufficient current strength scans the free surface of the photoconductive layer according to a given frame and brings said surface pointwise at the potential of the cathode which is termed zero volts. Be-

tween two successive scans, the potential of each point of the free surface of the photoconductive layer increases under the influence of a positive potential which is applied to the signal layer and under the influence of photoconductivity which is produced in the photoconductive layer by the optical image projected thereon. Each point, or more exactly each elementary surface element, of the photoconductive layer together with the signal layer constitutes a capacitor. The charge of said capacitor is periodically replenished by the scanning electron beam, for which more charge is required according as more light impinges upon the relevant point. The current which consequently flows through the connection of the signal layer comprises the information of the projected image as a function of time.

The current strength of the electron beam must be sufficiently large to provide elementary capacitors which are strongly discharged as a result of large current intensity, with sufficient charge. As soon as the free surface of the photosensitive layer is reduced to zero volts in a given point, the electrons of the electron beam can no longer reach said point. Their velocity becomes zero and then they are accelerated in the reverse direction and constitute the socalled return beam. Said return beam also experiences the influence of the deflection fields and scans the surface of the diaphragm facing the photoconductive layer. A part of the secondary electrons produced on the diaphragm has substantially the same kinetic energy as the electrons of the return beam and constitutes a secondary beam which together with the original (primary) electron beam scans the photoconductive layer but in a place different from the primary electron beam because the secondary beam is formed by electrons which have traversed the deflection fields three times instead of once. As a result of this an interfering signal is formed which is visible in the picture to be displayed.

According to the invention and in order to prevent the detrimental effect of the return beam, in a television camera tube of thetype mentioned in the first paragraph, the surface of the diaphragm facing the photoconductive layer, in so far as it is not present in the immediate proximity of the axis of the tube, encloses an acute angle with the direction of the said axis. This is preferably realized by giving the diaphragm the shape of a truncated cone in the narrow side of which the aperture is provided.

As a result of the invention, the secondary beam has a major direction which is not directed towards the photosensitive layer because a very considerable part of the produced secondary electrons has a direction which coincides in the same manner with the direction a of the primary electrons and with the normal to the surface of the diaphragm as this is the case upon reflection oflight rays (the angle of incidence is equal to the angle of reflection).

The invention will be described in greater detail with reference to the accompanying drawing of an embodiment of a television camera tube according to the invention.

The television camera tube shown has an evacuated glass envelope 1 with connection pins 2, an electron gun 3 with a cathode 4, a grid 5 and a anode 6, a focusing lens 7 consisting of the cylindrical electrodes 8, 9 and 10, a gauze-shaped electrode 11 and a photocon ductive layer 12 which is provided on a transparent, conductive signal layer 13, which signal layer 13 is provided on the window 14 of the tube. The cylindrical electrode 8 comprises a diaphragm 15 having an aperture 6 which restricts the cross-section of the electron beam produced by the electron gun 3. As a result of this, aberrations of the electron beam as a result of the focusing lens 7 are reduced and the electron beam scans the photosensitive layer 12 with a very small spot. This scanning occurs under the influence of deflection coils around the envelope 1 which are not shown. The supports of the electrodes in the tube and their connections to the connection pins 2 are not shown either. Perpendicular landing of the electron beam on the photoconductive layer 12 takes place under the influence of the electric field between the cylindrical electrode 10 and the gauze-shaped electrode 11. The diaphragm 15 is conical with the exception of a small part 17 which comprises the aperture 16. The return beam which returns from the photoconductive layer 12 and is formed by electrons which could no longer reach said layer, scans the diaphragm 15 also under the influence of the deflection coils, which is denoted by the arrow 18. As a result of the conical shape of the diaphragm 15, the secondary beam has a major direction which is denoted, for example, by the arrow 19. t

For the operation of this tube the following voltages may be used:

Cathode 4 1 O V grid 5 (during scanning) 40 V Anode 6 and electrode 8 300 V Electrode 9 V Electrode l0 300 V Gauze-shaped electrode ll 575 V Signal layer 13 45 V The potential of the surface of the photosensitive layer 12 which is scanned by the electron beam is periodically reduced to 0 volts. Between two successive scans said potential increases as a result of the photoconductivity in the layer 12 which is produced by an image projected on said layer and by the potential of 45 V of the signal layer 13. The parts of the free surface of the photoconductive layer 12 which are not scanned because they fall outside the rectangular frame, have a potential of nearly 45 V. As a-result of this the secondary beam can easily land on said parts. The interfering signal as a result of the secondary beam is thus mainly produced at those instants at which the secondary beam passes the non-used parts of the photoconductive layer 12. Since the photoconductive layer 12 is circular and the frame is rectangular, said parts have the shape of four circle segments. The secondary beam has traversed the deflection field three times and the primary beam only once. The frame of the secondary beam is therefore approximately three times as large 4 as the frame of the primary beam. The result of this is that, in particular in the case of uniform and weakly exposed scenes, a dark rectangle is visible in the displayed television picture the size of a third of the picture surrounded by four brighter circle segments. Said disturbing effect is prevented with a television camera tube according to the invention.

Since the return beam has traversed the deflection fields two times the diaphragm 15 is scanned by the return beam according to a raster which is twice as large as the frame of the primary beam on the photoconductive layer 12. The part 17 of the diaphragm 15 still causes a secondary beam with a major direction to wards the photoconductive layer 12 and must be chosen to be so small that said secondary beam remains within the part of the photoconductive layer 12 scanned by the primary beam since said part has a potential in the proximaty of 0 volts so that the secondary beam can land there with difficulty. ln the tube shown, the photoconductive layer 12 has a diameter of approximately 10 mm and the frame of the primary beam forms a rectangle of6 X 8 mm. The diameter of the part 17 of the diaphragm must then be significantly smaller than of the smallest dimension of the frame scanned by the primary beam, so significantly smaller than 4 mm, and in the example shown it is 2 mm.

What is claimed is:

l. A television camera tube comprising an evacuated envelope having a radiation transparent end wall, a transparent electrically conductive layer on said end wall, a photoconductive layer on said electrically conductive layer on which a potential distribution is.

formed on the surface thereof remote from said electrically conductive layer by projecting an optical image on said layer, an electron gun having a cathode spaced from and axially aligned with said photoconductive layer for producing an electron beam, deflection means for scanning said photoconductive layer with the electron beam for periodically reducing the potential distribution on the surface of said photoconductive layer to cathode potential, and a hollow cylindrical electrode between the electron gun and the photoconductive layer, said hollow cylindrical electrode having a diaphragm integral therewith with an aperture therein for restricting the crosssection of the electron beam, the surface of said diaphragm facing said photoconductive layer extending away from said layer and forming an acute angle with the axial direction for deflecting returned electrons from' said photoconductive layer, said diaphragm being a truncated cone having its smaller end nearer the photoconductive layer.,

(5/69) UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent 2 R66 079 Dated Februarv 11. 1975 Inventor(5) THEODORE G.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

a F" "I On the title page insert [30] Foreign Application Priority Data 0 July 2, 1971 Netherlands ..71o914o-- Signed and Scaled this 9 sixteenth Day Of September 1975 [SEAL] Attest:

RUTH C. MASON C. MARSHALL DANN Alresting Officer (ommissinner uj'Parems and Trademarks 22 2 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTIQN Patent No. 3,866,079 Eebmary lL 1915 Inventoflflw It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

On the title page insert -[73] Ass ignee: U.S. Philips Corporation,

New York, N.Y.--

. JSigned and sealed .this 13th day of May, 1975. I i

(SEAL) Attest:

C. MARSHALL DANN RUTH C. MASON Commissioner of Patents and Trademarks Attesting Officer 

1. A television camera tube comprising an evacuated envelope having a radiation transparent end wall, a transparent electrically conductive layer on said end wall, a photoconductive layer on said electrically conductive layer on which a potential distribution is formed on the surface thereof remote from said electrically conductive layer by projecting an optical image on said layer, an electron gun having a cathode spaced from and axially aligned with said photoconductive layer for producing an electron beam, deflection means for scanning said photoconductive layer with the electron beam for periodically reducing the potential distribution on the surface of said photoconductive layer to cathode potential, and a hollow cylindrical electrode between the electron gun and the photoconductive layer, said hollow cylindrical electrode having a diaphragm integral therewith with an aperture therein for restricting the crosssection of the electron beam, the surface of said diaphragm facing said photoconductive layer extending away from said layer and forming an acute angle with the axial direction for deflecting returned electrons from said photoconductive layer, said diaphragm being a truncated cone having its smaller end nearer the photoconductive layer. 